A power supply in electronic devices such as a television, a smart phone and a PC is required to supply a constant direct-current voltage in a stable manner. As such a power supply, a resonance-type switching power-supply device has been used.
The resonance-type switching power-supply device uses semiconductor elements such as MOSFET (metal-oxide-semiconductor field-effect transistor), IGBT (Insulated Gate Bipolar Transistor) and thyristor as a switching element and once converts the input DC voltage into AC voltage by the turn on-and-off of the switching element. The converted AC voltage is converted into a stable DC voltage sequentially through a transformer, a rectifier circuit and a smoothing circuit and then is outputted.
In such a switching power-supply device, the output voltage is stabilized by controlling a switching frequency of the switching element according to the output voltage. A control range of the switching frequency has a normal control range of a frequency range higher than a resonance frequency of a resonance circuit consisting of a capacitor and an inductor. In order to widen the control range of the output voltage as far as possible, there is a case where it is desired to cause a lower-limit frequency of the switching frequency to be close to the resonance frequency as far as possible. However, in this case, there is a possibility that a so-called off-resonance occurs if variation exists in the characteristics of the elements constituting a circuit. The so-called off-resonance means that the switching frequency is deviated from the normal range and becomes lower than the resonance frequency.
Therefore, in a switching power-supply device disclosed in Japanese Patent Laid-Open Publication H9-308243, the off-resonance is suppressed by performing a control to return the switching frequency to the normal control range when it is detected that the switching frequency is deviated from the normal control range.